Potential for Subsidence at the Low-level Waste Disposal Area

Abstract

U.S. Department of Energy (DOE) Order 435.1, Radioactive Waste Management requires that DOE low-level radioactive waste (LLW) disposal facilities receive a Disposal Authorization Statement (DAS) from DOEHeadquarters. The DAS for the LLW disposal facility at the Radioactive Waste Management Complex (RWMC) at the Idaho National Engineering and Environmental Laboratory (INEEL) was granted in April 2000 and included a number of conditions that must be addressed. A maintenance plan (Schuman 2000) was prepared that identifies the tasks to be completed to address the conditions in the DAS as well as a schedule for their completion. The need for a subsidence analysis was one of the conditions identified for the DAS, and thus, a task to prepare a subsidence analysis was included in the maintenance plan. This document provides the information necessary to satisfy that requirement. (Schuman 2000) was prepared that identifies the tasks to be completed to address the conditions in the DAS as well as a schedule for their completion. The need for a subsidence analysis was one of the conditions identified for the DAS, and thus, a task to prepare a subsidence analysis was included in the maintenance plan. This document provides the information necessary to satisfy that requirement.A site-specific performance assessment (PA) and composite analysis (CA) are critical parts of the documentation forming the basis for the DAS. The PA and CA estimate the potential impacts of the disposed waste on human health and safety, and provide information regarding disposal operations, facility design, and closure required to ensure that potential risks are maintained at acceptable levels. The PA focuses on the wastes placed in the active LLW disposal facility, so that design and operational controls may be established to ensure that performance objectives will be met. The CA provides a reasonably conservative assessment of the cumulative impacts from the active LLW disposal facility and all other sources of radioactive contamination that could interact with releases from the LLW disposal facility. The RWMC PA (Case et al., 2000) and CA were completed and approved by DOE-ID. The documents were then submitted to the DOE-Headquarters Low-Level Waste Disposal Facility Federal Review Group (LFRG) in September 2000.LLW disposal facility design constraints are established based on the results of the PA and CA and the ability to meet the design constraints could be influenced by subsidence. For example, design constraints from the PA and CA that may be influenced by subsidence include:• the need to maintain a long-term average infiltration rate of less than or equal to 1 cm/yr, and• the need to maintain a final cover thickness of at least 2.4 m (8 ft) over the waste in the pits and 3.3 m (10 ft) over the soil vaults.Thus, the LFRG considered it important to address the potential for subsidence at the RWMC Subsurface Disposal Area (SDA) LLW disposal facility in the context of the DAS.The LLW disposal facility at the RWMC is somewhat unique i...